At present there is great interest in expanding the scope of modern technology into the sphere of personal security. In response to this interest, the capabilities of mobile communication systems such as cellular telephone systems can be enhanced by enabling wireless communication devices such as cellular telephones to place spoken or silent emergency calls to a "911" emergency response center. A person equipped with such a wireless communication device can summon help in case of assault, burglary, or fire, when a conventional wireline telephone is out of reach or out of service.
In this vein, U.S. Pat. No. 5,388,147, "Cellular telecommunication switching system for providing public emergency call location information" and U.S. Pat. No. 5,479,482, "Cellular terminal for providing public emergency call location information" teach combining a cellular telephone and a Global Positioning System (GPS) receiver so that the phone's approximate physical location, i.e., its approximate geocoordinates, can be reported as part of the emergency call. For the convenience of those who respond to the call, the emergency response center--which is called herein a Public Safety Answering Point (PSAP)--converts these approximate geocoordinates into an approximate location municipal street address.
As a cost-saving alternative to the use of GPS technology, the cellular system itself can be equipped with ancillary equipment that enables it to deduce the cellular telephone's position by estimating signal propagation times between the cellular telephone and various terrestrial receivers sometimes located at cellular base stations. This approach to locating the cellular telephone is called here cellular positioning. Adopting cellular positioning rather than GPS eliminates the need to equip each cellular telephone with a GPS receiver, and therefore promises to offer economic advantage. Moreover, radio frequency links provided by cellular systems are more robust than links provided by satellite systems such as GPS, and cellular links may therefore be expected to penetrate deeper into buildings and further behind radio-frequency shadows. Nevertheless, cellular positioning has the disadvantage of being generally less accurate and less capable of fine positioning resolution than GPS positioning.
Various aspects of emergency-response systems based on cellular positioning have been disclosed in U.S. Patent Applications to the present assignee. These applications are: U.S. patent application No. 08/839,859 "Systems and methods for handling emergency calls in hierarchical cell structures;" U.S. patent application No. 08/843,752 "Systems and methods for increasing emergency call access speed in radiocommunication systems;" U.S. patent application 08/839,860 "Systems and methods for identifying emergency calls in radiocommunication systems;" U.S. patent application No. 08/839,864 "Systems and methods for locating remote terminals in radiocommunication systems;" U.S. patent application No. 08/844,883 Systems and methods for providing information to emergency service centers;" U.S. patent application No. 08/838,027 "A cellular positioning systems that compensates for received signal delays in positioning radio receivers;" U.S. patent application No. 08/840,590 "Systems and methods for locating remote units operating in a radiocommunication system using an adjunct system;" and U.S. patent applicaiton No. 08/839,861 "Interaction between an adjunct positioning system and a radiocommunication system."
Regardless of the kind of positioning technology employed, both the determination of the approximate geocoordinates and their conversion to an approximate location municipal street address are subject to process errors, and combining these two process errors may result in an incorrect or incomplete conversion of the geocoordinates. This is especially troublesome in densely populated residential areas such as urban apartment buildings, where, for example, bedrooms of different residences may be nearly contiguous, being separated only by a few inches of wall, or separated mainly in the vertical dimension rather than the horizontal. In such cases it is unclear which apartment the emergency responder should go to.
Thus, the possibility of error and uncertainty limits the extent to which a cellular telephone or other wireless communication device can be used for silent at-home security applications, particularly when cellular positioning is employed. Consequently, there remains a need to improve the positioning accuracy of a wireless communication system to the extent that the at-home location of a wireless communication device can be identified with certainty, so that the wireless communication device can serve better when placing emergency calls such as medical alerts for homebound users unable to reach a wireline terminal, last-resort silent burglar alarms invulnerable to attacks on telephone lines, fire alarms, and the like.